The present invention relates to a solid acid catalyst and to a method for producing an ester, a ketal or an acetal by using the solid acid catalyst.
A solid acid catalyst has an advantage such as an easy operation for isolating a product after the reaction. Therefore, it is has been examined from the viewpoint of ability for use in various organic reactions. For example, there are known a zeolite (JP-A 61-200943), a silicate of the IV group elements (EP0623581A2), hydrous zirconium oxide (JP-B 4-28250) and the like for producing an ester and, then, an organic solid catalyst such as an ion-exchange resin and an inorganic solid catalyst such as hydrous zirconium oxide (JP-A 63-146838) and mordenite (JP-A 60-188338) for producing an acetal or a ketal. Those described in U.S. Pat. No. 4,202,798 and U.S. Pat. No. 4,251,350 are known.
In case a solid catalysts selected therefrom are used especially in a liquid phase reaction, the activity or the durability of the strength of formed catalyst can not satisfy, because of insufficient activity at a low temperature, elution of a catalytic component into the liquid phase, and the like.
It is an object of the present invention to provide a solid acid catalyst which develops its high activity and exhibits its high durability in various organic reactions.
The present invention relates to a solid acid catalyst having the following structure (A), structure (B) and metal atom (C) and to a method for producing an ester, a ketal or an acetal by using the solid acid catalyst.
Structure (A): a structure in which a hydrogen atom is eliminated from at least one of OH groups contained in an inorganic phosphorus acid;
Structure (B): a structure in which a hydrogen atom is eliminated from at least one of OH groups contained in an organic phosphorus acid represented by the formula (1) or (2); 
wherein each of xe2x80x94R1 and xe2x80x94R2 is selected from xe2x80x94R, xe2x80x94OR, xe2x80x94OH and xe2x80x94H in which at least one of xe2x80x94R1 and xe2x80x94R2 is xe2x80x94R or xe2x80x94OR, given that xe2x80x94R is an organic group having 1 to 22 carbon atoms;
Metal atom (C): at least one atom selected from aluminum, gallium and iron.
In this description, the structure (A) is derived from a phosphorus-containing compound (phosphorus acid) such as phosphonic acid, phosphorous acid, phosphinic acid, phosphoric acid, orthophosphoric acid and metaphosphoric acid, and other isomers or derivatives thereof. The phosphorus acid may be an oxy acid of phosphorus.
Further, the present invention provides the solid acid catalyst as mentioned above, wherein the structure (A) is a structure derived from orthophosphoric acid.
Furthermore, the present invention provides the solid acid catalyst as mentioned above, wherein the structure (B) is a structure derived from phosphonic acid.
Then, the present invention provides a method for producing an ester, which comprises producing an ester from a raw ester or raw carboxylic acid (a) and a raw alcohol (b) by using the catalyst as defined above.
The present invention provides also a method for producing a ketal or an acetal, which comprises producing a ketal or an acetal from a ketone or aldehyde (c) and an alcohol (d) by using the catalyst as defined above.
The present invention provides a method for preparing a solid acid catalyst having the following structure (A), structure (B) and metal atom(C);
Structure (A): a structure in which a hydrogen atom is eliminated from at least one of OH groups contained in an inorganic phosphorus acid;
Structure (B): a structure in which a hydrogen atom is eliminated from at least one of OH groups contained in an organic phosphorus acid represented by the formula (1) or (2): 
wherein each of xe2x80x94R1 and xe2x80x94R2 is selected from xe2x80x94R, xe2x80x94OR, xe2x80x94OH and xe2x80x94H in which at least one of xe2x80x94R1 and xe2x80x94R2 is xe2x80x94R or xe2x80x94OR, given that xe2x80x94R is an organic group having 1 to 22 carbon atoms;
Metal atom (C): at least one atom selected from aluminum, gallium and iron.
The present invention provides use of the catalyst as defined above for production of an ester compound.
The present invention provides use of the catalyst as defined above for production of a ketal compound or an acetal compound.
The catalyst of the invention may be that being obtainable from the inorganic phosphorus-containing compound for the structure (A), the organic phosphorus acid for the structure (B) and a metal compound for the metal (C).
In the structure (A) in the solid acid catalyst of the present invention, the inorganic phosphorus acid includes orthophosphoric acid, metaphosphoric acid and a condensed phosphoric acid such as pyrophosphoric acid. Orthophosphoric acid is preferable from the viewpoint of performance.
Also, in the structure (B), the organic phosphorus acid represented by the formula (1) or (2) includes phosphonic acid, phosphonic acid monoester, phosphinic acid, phosphoric acid monoester, phosphoric acid diester, phosphorous acid monoester and phosphorous acid diester. A mixture thereof may be included. Phosphonic acid is preferable.
The organic group xe2x80x94R in the organic phosphorus acid is preferable to be an alkyl group such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, n-hexyl, 2-ethylhexyl, octyl, dodecyl and octadecyl or to be an aryl group such as phenyl and 3-methylphenyl. Also, one of these groups may be combined with an amino group, an alkoxy group, a carbonyl group, an alkoxycarbonyl group, a carboxyl group, a halogen group including chloro group, phosphono group, sulfo group or the like for use.
The metal atom (C) is preferably aluminum from the viewpoint of performance and/or cost. Then, it may contain a small amount of a metal atom other than aluminum, gallium and iron in order to improve selectivity or other performances.
From the viewpoint of performance, the ratio of the structure (A) to the structure (B) in the solid acid catalyst of the present invention is preferable in such ratio that the molecular ratio x of an organic phosphorus acid represented by the following formula (3) is 0.01 to 0.99. Here, [Pinorg] shows the molecular number of phosphorus atom contained in the structure (A) and [Porg] shows the molecular number of phosphorus atom contained in the structure (B).
x=[Porg]/([Pinorg]+[Porg])xe2x80x83xe2x80x83(3)
Then, a more preferable ratio is present in the ratio x depending upon a reaction to be utilized. The x is preferably 0.01 to 0.7 and more preferably 0.01 to 0.5 at the production of an ester. On the other hand, the x is preferably 0.05 to 0.99 and more preferably 0.1 to 0.8 at the production of a ketal or acetal.
Then, the value Y represented by the following formula (4) is preferably 0.15 to 2.0.
Y=[Metal]/([Pinorg]+[Porg])xe2x80x83xe2x80x83(4)
Here, [Metal] shows the number of the metal atoms (C) in the catalyst.
All of the metal atom (C) contained in the catalyst doesn""t bond necessarily to the structure (A) or (B). Some of the metal atom (C) may be present in the form of a metal oxide, metal hydroxide or the like.
As the method for preparing the solid acid catalyst of the present invention, a precipitating method, a method for impregnating an inorganic phosphorus acid and an organic phosphorus acid onto a metal oxide or a metal hydroxide, and a method for substituting an inorganic phosphorus acid group in an inorganic phosphorus acid aluminum salt-gel for an organic phosphorus acid group or the like is used. The precipitating method is preferable. Then, the method for preparing the solid acid catalyst may comprise merely mixing a powder of an inorganic phosphorus acid salt with another powder of an organic phosphorus acid salt, as it is, homogeneously. Here, the powder of the inorganic phosphorus acid salt may comprise the above-mentioned structure (A) and the metal atom (C), and the other powder of the organic phosphorus acid salt may comprise the above-mentioned structure (B) and the metal atom (C). In the precipitating method, a precipitate of the present catalyst is obtained by mixing an aqueous solution (S) of a water-soluble salt of aluminum, an inorganic phosphorus acid and an organic phosphorus acid with an alkali (T) such as an aqueous solution of ammonia, an aqueous solution of sodium carbonate and an aqueous solution of sodium hydroxide and then regulating the pH. Also, there is no limit of order for adding the aqueous solution (S) and the alkali (T). Further, in case the organic phosphorus acid has poor water-solubility, the aqueous solution (S) may be prepared by properly adding a solvent such as methanol and acetone. The obtained precipitate is dried and, if necessary, further calcinated. At this time, calcination at a high temperature causes possibility of losing an organic group in the structure derived from the organic phosphorus acid. Hence calcination at a temperature of 600xc2x0 C. or lower is preferable and at a temperature of 500xc2x0 C. or lower is more preferable.
Also, it is possible to obtain a catalyst supported on a carrier by making the carrier having its large surface-area coexist at when the catalyst of the present invention is prepared. As the carrier, silica, alumina, silica-alumina, titania, zirconia, an activated carbon or the like may be used. The use of excessive carrier decreases the content of an active component to decrease the activity. Therefore, the proportion of the carrier in the catalyst is preferably 90% by weight or less.
It is possible that the form of the catalyst is powder for use, as it is, to disperse in a raw material. On the other hand, it is possible that the form of the catalyst is formed into a proper granule or shape and charged in a reaction-column to carry out the continuous reaction.
It is necessary and essential that the solid acid catalyst of the present invention has the structures (A) and (B) simultaneously in the condition under which the catalyst is used for a reaction. However, it is unnecessary to have all of the inorganic phosphorus acid and organic phosphorus acid which have been used for the preparation.
The solid acid catalyst of the present invention can be utilized in various catalytic reactions. For example, the catalytic reaction includes a transesterification, an esterification, a formation of an acetal from an aldehyde and an alcohol or a formation of a ketal from a ketone and an alcohol, an aldol condensation, an amidation, an amination, a synthesis of an olefin or ether by dehydrating an alcohol or alcohols, a hydration of an olefin and an addition to olefin with an alcohol, an isomerization and an alkylation or acylation of an aromatic ring, and the like.
As to each of reaction-conditions in the various reactions mentioned above, for example, the optimum condition of the reaction phase (vapor phase or liquid phase) is selected according to the type of reaction. Since the solid acid catalyst of the present invention exhibits its high activity even in a mild condition, it is preferably used not only in a vapor phase but also in a liquid phase-condition where a conventional solid catalyst develops its high activity with difficulty. Also, for example, the optimum condition of the reaction-pressure is selected in accordance with the type of reaction. The reaction may be carried out under pressurization, atmospheric pressure or reduced pressure. Also, the reaction-temperature is preferable not to lose the organic groups in the structure (B), for instance. For example, in the presence of oxygen, the catalyst is used preferably at 600xc2x0 C. or less and more preferably at 500xc2x0 C. or less.
In a transesterification or an esterification, the solid acid catalyst of the present invention exhibits very high activity and good selectivity and, further, can maintain its activity and selectivity for a long period. As the result, a highly pure ester can be obtained in a short reaction-time with high yield over a long term.
A transesterification or esterification is carried out by mixing a raw ester or raw carboxylic acid (a) with a raw alcohol (b) and bringing the resultant mixture into contact with the solid acid catalyst of the present invention under a reaction-condition.
As the raw ester among the raw ester or raw carboxylic acid (a), an ester or partial ester, for example, of a C1-22 linear or branched aliphatic carboxylic acid or aromatic carboxylic acid or a mixture thereof with a C1-22 linear or branched monohydric or polyhydric alcohol is used. Specifically, such an ester or partial ester is produced from a carboxylic acid or dicarboxylic acid such as acetic acid, butyric acid, caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, oleic acid, stearic acid and benzoic acid or a mixture thereof with a monohydric aliphatic alcohol such as methanol, ethanol, propanol, butanol, octanol and stearyl alcohol; a monohydric aromatic alcohol such as phenol or a polyhydric alcohol such as ethylene glycol, propylene glycol, glycerol, pentaerythritol and sorbitol. It is preferable to be a natural vegetable (plant and so on) oil such as a monoglyceride, a diglyceride, a triglyceride, coconut oil, palm oil and palm-kernel oil or an animal oil such as beef-tallow and pork-lard. Also, as the raw carboxylic acid, the above-mentioned C1-22 linear or branched aliphatic carboxylic acid or aromatic carboxylic acid or a mixture thereof is used.
Also, as the raw alcohol (b), the above-mentioned C1-22 linear or branched monohydric or polyhydric alcohol is used.
The esterifying method includes, for example, those in which the raw ester or raw carboxylic acid (a) and the raw alcohol (b) are continuously fed into a reaction-column charged with a solid acid catalyst and those in which the raw ester or raw carboxylic acid (a) and the raw alcohol (b) are bought into contact with a solid acid catalyst in a reaction-chamber to react in batch-wise. At this time, even only filtration satisfies a treatment after the reaction. Therefore, a neutralizing step and catalyst-removing step which are required in the case of using a homogeneous catalyst used in general can be omitted. Also, the recovery of un-reacted raw materials is made easy.
With regard to the reaction-pressure and temperature, a preferable condition can be selected according to the objective reaction. As to the reaction-temperature, it is preferable to carry out a reaction at a lower temperature in case of taking consideration of a side reaction. The solid acid catalyst of the present invention exhibits higher activity, as compared with a conventional solid acid catalyst, in an esterification even at a lower temperature-condition. Therefore, it is possible to obtain an objective ester at high selectivity.
The solid acid catalyst of the present invention also exhibits very high activity and good selectivity in an acetal- or ketal-formation of the ketone or aldehyde (c) and the alcohol (d) and further can maintain its activity and selectivity for a long period. As the result, a highly pure acetal or ketal can be obtained in a short reaction-time with high yield over a long term.
As the ketone or aldehyde (c), a C1-22 linear or branched aliphatic ketone or aldehyde, or an aromatic ketone or aldehyde or the like is used. More specifically, acetone, methyl ethyl ketone, 3-pentanone, 2-pentanone, di-n-hexyl ketone, cyclohexanone, acetophenone, acetaldehyde, propionaldehyde, n-hexanal, n-dodecanal, benzaldehyde and the like may be exemplified.
Also, as the alcohol (d), the same alcohol as the raw alcohol (b) exemplified for the above-mentioned esterification is used.
A ketal- or acetal-forming method includes, for example, those in which the ketone or aldehyde (c) and the alcohol (d) are continuously fed into a reaction-column charged with a catalyst, those in which the reaction is carried out in a reaction-chamber in batch-wise and so on. At this time, a proper azeotropic solvent such as benzene and heptane is used to distill away the generated water, that may make the reaction-rate or the equilibration of the reaction advantageous.
If the solid acid catalyst of the present invention is used, even only filtration satisfies a treatment after the reaction. Therefore, a neutralizing step which is required in the case of using a homogeneous catalyst, such as paratoluenesulfonic acid and a mineral acid, used in general can be omitted. Also, in the distillation, no side reaction and the like due to the presence of a salt takes place. Therefore, the yield in a refining time can be improved and further the recovery of an un-reacted raw material is made easy.
The solid acid catalyst of the present invention has an organic group on the surface of the solid. Therefore, the active site on the surface of the catalyst has high affinity to an organic reactant and hence the catalyst is highly active.
This catalyst may be utilized in various catalytic reactions. Particularly, for producing an ester by esterification or transesterification and for producing a ketal or acetal by an acetal- or ketal-forming reaction, the catalyst can be easily separated from the products and makes it possible to obtain the product with high yield at high selectivity over a long term.